1. Field of the Invention
The invention concerns charge transfer matrix photodetectors of the kind having filtering means to give, at output, only those useful or necessary charges that correspond to a useful portion of an incident light radiation.
2. Description of the Prior Art
Photodetectors are used, in principle, to store, transfer and deliver charges at output, according to the incident light radiation to which they are subjected. They do this to restore a digital image. Among photodetectors, we might distinguish those generally having a single row (or line) of photosensitive elements and those (surface or matrix type photodetectors) that are organized either in frame transfer mode, or in inter-line transfer mode, and are formed by sets of parallel charge transfer channels. The invention concerns only matrix photodetectors.
FIG. 1 gives a schematic view of a standard matrix photodetector organized in frame transfer mode. This photodetector 1 has a first zone 1, having a photosensitive sub-zone 10, commonly called an image zone, and a non-photosensitive sub-zone 11, commonly called a memory zone. The photodetector 1 also has a second zone 2, connected to the first zone 1, having a read-out buffer or register 20, extended by a read diode 21, connected to a control transistor 22.
The image zone 10 is a matrix of n lines and p columns. Each column is a vertical charge transfer shift register forming a charge transfer channel. The compartments of the register are photosensitive, and charges collect therein, depending on the illumination of the compartment. The memory zone is formed identically to the image zone, but the compartments of the different registers are not photosensitive. The read-out buffer is a horizontal shift register.
The image zone and the memory zone also have transfer gates which surmount the transfer channels and are capable of receiving low or high control signals to apply a low or high potential level to the transfer channels, so as to enable the transfer of charges therein. The read-out buffer is capable of receiving at least one part of the charges stored in the memory zone, and is surmounted by a read gate capable of also receiving low or high control signals to apply a low or high potential level to the read-out buffer, so as to enable the transfer of the charges, contained in this read-out buffer, into the read diode.
FIG. 2 gives a schematic illustration of a matrix photodetector organized in inter-line transfer mode. The image zone 10' is formed by columns of photodiodes interconnected with columns of photosensitive compartments, belonging to the vertical shift registers forming the memory zone 11'. The low part of each transfer channel CT' is connected to a compartment of a horizontal shift register forming the read-out buffer 20'.
A brief description shall now be given of the working principle of a matrix photodetector organized in frame transfer mode. During an integration period, charges corresponding to the light image received are stored in the registers of the image zone 10. When this integration period has elapsed, the full image stored in the image zone 10 is transferred, during a so-called transfer period, into the memory zone 11. This transfer is done line by line, by a vertical shift. During the next integration period, while the next image is stored in the image zone, the image stored in the memory zone is transferred, line by line, into the read-out buffer by vertical shift, and each line is transferred from the read-out buffer to the read diode by means of a horizontal shift, and then from the read diode to the output.
The working principle of a photodetector organized in inter-line transfer mode is substantially similar. The charges stored in the photodiodes are transferred horizontally towards the shift registers, then each line of the shift registers is shifted vertically into the read-out buffer and transferred to the read diode by a horizontal shift of the latter.
In many cases when pictures are taken, only one part of the image is truly useful or necessary at a given moment. For example, when an object is being tracked, once it has been identified, its tracking needs only low resolution but a high rate of renewal of the image.
FIGS. 3a to 3c give a schematic view of a filtering principle currently used in the prior art to deliver, at output, only useful charges corresponding to said useful portion IMD of the incident light radiation IM, namely that portion of the image which it is desired to display. FIG. 3A shows the desired image IMD. On the n lines of the total image IM, it is desired to eliminate the lines 1 to i as well as the lines j+1 to n so as to display only the lines i+1 to j. In FIG. 3B, the total image IM, including the desired image IMD, is shown as having been already transferred into the memory zone 11. Under the effect of the control signal SCM, which may be low or high, applied to the different transfer gates of the memory zone, each line of this memory zone is transferred into the read-out buffer. This read-out buffer, under the effect of the control signal SCL, which may be low or high, applied to the read gate, transfers the line that it contains into the read diode. This line is then delivered, at output, under the control if the filtering control signal SCF which is actually a control clock signal of the control transistor gate. Thus, when the control signal SCF has a change in state (high/low or reverse) during the action of the control signal SCL, the line present in the read-out buffer is effectively transferred into the read diode and then delivered at output whereas, when the control signal SCF is at the high state, the read diode is reset and no line is delivered. The control transistor 22 and the control signal SCF are therefore part of the filtering means.
A timing diagram of the restitution of the image, present in the memory zone, is shown in FIG. 3C. To explain this timing diagram, the read-out buffer is assumed to have only one line. During the i first pulses of the control signal SCM, the i first lines of the image IM are successively transferred into the read-out buffer 2. This read-out buffer, under the effect of the i first pulses of the control signal SCL, releases these lines. Since the control signal SCF is, during this period, constantly at the high state, none of these lines is actually delivered at output of the photodetector. Hence the unwanted charges, corresponding to the lines 1 to i of the image, have been eliminated. During the pulses i+1 to j of the control signal SCM, these corresponding lines are transferred into the read-out buffer which, under the effect of the corresponding pulses i+1 to j of the control signal SCL, makes the corresponding lines accessible to the outside. Since, during the same period, the control signal SCF has a change in state during the transfer of a line outside the read-out buffer; these lines are effectively delivered at the output of the photodetector, and the lines i+1 to j of the desired image are restored. During the pulses j+1 to n of the control signals SCM and SCL, the control signal SCF is kept at the high state, and the unwanted charges are thus eliminated from the lines j to n. The process is repeated for the following image. It can be seen, at once, that the image-grabbing speed is related to the speed with which the unwanted lines are eliminated by the read-out buffer. At present, to increase this rate, several lines at a time are summed in the read-out buffer. However, this efficiency is limited by the size of the read-out buffer, depending on whether it may contain two or n lines simultaneously. The maximum number of lines that can be contained in a read-out buffer is between four to eight for, beyond this figure, the number becomes too great and there is the risk that the reading frequency might deteriorate. The data rate is thus limited by the size of this read-out buffer, and this has adverse implications for very high data rates.
The present invention overcomes these drawbacks by proposing a matrix photodetector provided with means to filter useful charges that require no oversizing of the read-out buffer and permit a very high image-grabbing rate.